The present invention generally relates to chemical vapor deposition and more particularly to a system and method for depositing tungsten selectively on a metal part or semiconductor part of a substrate while prohibiting growth of tungsten layer on an insulator part.
Conventionally, chemical vapor deposition systems using a resistance heater as means for heating a substrate has been employed for depositing metals such as tungsten or semiconductors selectively on the substrate. In such a conventional system, not only the substrate alone but other part of the system in a reaction chamber such as a holder of the substrate or the heater itself are heated relatively uniformly, and associated therewith, there arises a problem in that byproducts of CVD reaction tends to be formed extensively on these heated parts. These byproducts tend to react with the insulator part of the substrate to form the nuclei of tungsten growth, and thereby, the selective growth of desired material such as tungsten on the substrate is no longer obtained.
In order to avoid this problem, localized heating of the substrate by infrared irradiation is commonly used. In the case of heating by the infrared irradiation, the substrate alone is heated and cooled rapidly and selectively while the holder or other parts in the reaction chamber of the CVD system are held relatively cool. However, the system using the resistance heater is supposed to be advantageous when the substrate is replaced one after another for mass production, because of the relatively stabilized temperature of the holder and the substrate.
FIG. 1 shows a conventional chemical vapor deposition system schematically.
Referring to FIG. 1, a source gas introduced into a reaction chamber 36 via an inlet tube 1 is released into the chamber through a shower nozzle 31. In the chamber 36, a substrate 33 is held on a holder 34 of quartz and the like, and the substrate 33 is heated via the holder 34 by a resistance heater 35 provided underneath the holder 34 at the outside of the reaction chamber 36. Further, the reaction chamber 36 is evacuated via an exhaust port 32 by an evacuation pump not illustrated as well as via another exhaust port 37a by an evacuation pump 37. When an infrared lamp is used for heating the substrate 33, the lamp may be provided underneath the holder 34 so as to irradiate the substrate 33 through the holder or may be provided above the substrate 33.
In the CVD system of FIG. 1, the shower nozzle 31 is separated from the wafer 33 by a distance of about 20 cm, and the source gas is introduced into the reaction chamber 36 after passing through a mesh structure in the shower nozzle 31 (not illustrated) such that the source gas uniformly covers the surface of the substrate 33.
Conventionally, the selective growth of tungsten by the CVD process has been generally undertaken at a relatively high total pressure such as 10.sup.-2 Torr or more. The evaluation of the property such as resistivity of the tungsten layer thus grown or corrosion taking place in the silicon substrate, is well documented.
However, when a resistance heater is used for the heat source 35 in particular, there arises a problem in that byproduct molecules such as SiF.sub.x are formed extensively at the heated surface of the holder 34 and the like as a result of the reaction of source gases such as WF.sub.6 and SiH.sub.4 and these byproduct molecules cause an unwanted reaction with the substrate 33 known as encroachment when the source gas, particularly WF.sub.6 is transported to the substrate 33. Thereby, the quality of the tungsten layer deposited on silicon is degraded. Further, the byproduct molecules react with the insulator part of the substrate and form the nuclei for growth of tungsten. Thereby, the selective growth itself is lost.
Conventionally, it is known that such undesirable problems can be avoided when the total pressure in the reaction chamber is reduced below 10.sup.-3 Torr during the CVD process. It is generally accepted that such a reduction of the total pressure results in an increase in the mean free path of the byproduct molecules which increase in the mean free path facilitates the scavenging of the reaction chamber. According to the experiments conducted by the applicants of the present invention, the selectivity of tungsten growth is certainly improved when conducted under the total pressure of about 10.sup.-3 Torr or less as compared with the case where the growth is made under the total pressure of 10.sup.-2 Torr or more.
However, such a growth of tungsten under the reduced total pressure has a problem in that the rate of growth is too slow because of the reduced supply of reactant species to the substrate. Further, the problem of encroachment of tungsten into silicon causing the degradation of the tungsten layer is not completely solved.